The present invention relates to a suspended front fork for a two-wheeled vehicle, and more particularly an all-terrain bicycle and motorcycle.
The front fork of such a vehicle has two essential functions:
1. Steering function: it must constantly provide for the steering and stability of the vehicle (bend, straight line, braking, acceleration).
2. Suspension function: this function is needed to ensure comfort, but also to keep the tyre in contact with the ground at all times.
The difficulty in designing such a fork arises in that the second function (suspension) involves a variation in geometry that is not always compatible with the first function (steering).
Such a compromise currently exists with the so-called telescopic fork.
This fork comprises a steering pivot and two tubular, telescopic branches parallel to the steering pivot. The upper parts of these branches are fixed to this pivot by means of two supports in the shape of a T. There is one T at each end of the pivot. Spring/shock-absorber combinations are contained in the two branches to provide the suspension function. The wheel axis is fixed to the lower ends of the two branches.
This type of fork can be found on 99% of motorcycles produced throughout the world and on over 80% of all-terrain bicycles with suspended forks.
Advantages of the telescopic fork:
1. Aesthetic appearance: it is mechanism which is very simple in appearance, the mechanical part being hidden inside the tubes.
2. Low manufacturing costs because of the simplicity of the mechanism.
3. Operation in keeping with the geometry of a two wheeled vehicle: the angle of the fork is such that the forces applied to it on braking or when the wheel passes over an obstacle, are virtually coaxial with the branches of this fork, resulting in low flexion.
4. A geometry which remains constant with the suspension travel, since the offset of the wheel axis with respect to the steering pivot is constant.
At constant attitude (without pitching), the trail rake is therefore constant. The latter only varies with variation in attitude (pitching): dive (depression of the front suspension, raising of the rear suspension) causes its reduction. Conversely, rearing causes its increase. During braking (therefore diving), the mass transfer onto the front makes the steering heavier, which should prevent the cyclist from turning at the same time as he is braking.
As the reduction in trail compensates for the increase in load on the front, the cyclist retains the ability to change direction easily.
Drawbacks of the telescopic fork:
1. Friction.
Plunger pistons must slide in the fork covers without play. There is therefore friction which is all the greater if the arms are subject to flexion forces.
Current motorcycle technology means that the forks are becoming more and more vertical (reduced by 10xc2x0 in 20 years) . The forks are therefore subjected to increasingly large flexion forces. Progress achieved by braking devices is also going in the same negative direction (more flexion force).
For all-terrain bicycles, variations in attitude are so big because of the small wheelbase, that the same problem is encountered, namely a fork which is very vertical under braking, leading to very high flexural stresses. In this context, telescopic forks are functioning increasingly badly.
2. The linearity of the suspension of the telescopic fork.
A front suspension is, much more than a rear suspension, subjected to major variations in load. On a motorcycle, a minimum load of zero kg passes easily to a maximum load of 300 kg. A linear suspension cannot operate correctly with such different loads (too hard for minimum forces and too soft for maximum forces).
3. Stiffness.
Various studies show this weakness. On a motorcycle, the torsional stiffness of a telescopic fork does not reach 6% that of the frame+engine. Its stiffness to longitudinal flexion is below 60%, its stiffness to transverse flexion barely reaches 35%.
4. Geometry.
The trail is determined by the rake angle (angle of the steering column with the vertical), the offset of the wheel axis and the circumference of the tyre. During operation of a telescopic fork, the trail varies according to a single variable, the rake angle (by variation of the attitude), but the amplitude in the variation of the trail with the attitude cannot be controlled.
Other forks:
The lever forks (above all for scooters) are selected because of their small height since they are well suited to small wheels. They have little amplitude, except for the fork known by the name EARLES which provides a large lever arm effect for a greater travel, but has a lot of inertia rotation around the steering pivot.
These levers generally have a push wheel, but there is pull wheel version. Remember that a pull wheel fork means that the arm is located in front of the wheel axis with respect to the direction of travel, the opposite being the case for a push wheel.
Parallelogram forks.
The parallelogram fork is the predecessor of the suspended fork for the motorcycle. Today it has been replaced by the telescopic fork. However, they were both invented at the beginning of the century. Parallelogram forks have disappeared from motorcycles, but several still exist for all-terrain bicycles. They are all of the push-wheel variety. They suffer from a small travel (80/90 millimeters maximum), and their theoretical advantages have not been seen in the field.
The purpose of the invention is to provide a suspended front fork which eliminates friction, has progressive suspension, has high stiffness with the option of adjusting the geometry and of varying the rake angle, in order to adapt them to the type of vehicle to which they will be fitted.
For this purpose, the fork to which the invention relates, of the type comprising at least one arm at the lower end of which is mounted the wheel axis and whose upper end is associated with the steering pivot, is characterized in that on the steering pivot and in front of the latter is fixed a rigid frame comprising at least one vertical flange, serving as a mount for two horizontal axes, each vertically offset with respect to the other, articulated to the ends of one upper and one lower small connecting rod, whose other ends are articulated to each arm of the fork in the upper third of the length of the arm, an elastic shock-absorbing element having one end fixed on the rigid frame and its other end fixed to one of the upper or lower small connecting rods.
According to one embodiment, rigid frame comprises two horizontal plates, one upper and one lower, fixed respectively to the upper and lower ends of the steering pivot, on the lateral edges of which plates the vertical flanges are fixed.
In a conventional embodiment, this fork comprises two parallel arms connected by a stiffening piece, each connecting rod splitting into two connected branches.
According to one characteristic of the invention, the axis articulating the upper connecting rod to the rigid frame is located between the two plates, while the axis articulating the lower connecting rod to the rigid frame is located below the lower plate.
In order to allow a large amount of travel without disturbing the conditions of use of a vehicle, the branches of the two connecting rods are set apart by a distance greater than the respective widths of the lower plate, the steering pivot and the shock-absorber element, in order to be able to pass on either side of these elements.
The branches of the connecting rods may be located inside or outside the vertical flanges.
According to a first option, the elastic shock-absorber element acts directly on one of the two connecting rods, on a part integral with the connecting rod in question, the axis articulating the shock-absorber to this part being offset with respect to the axis of the connecting rod, so that, in the position of maximum compression, the line passing through the axis articulating the shock-absorber and through the axis articulating the connecting rod in question to the rigid frame, are almost perpendicular to the axis of the shock-absorber.
It is important to note that this arrangement allows the damping effect to be a progressive one.
According to another embodiment of this fork, the lower end of the elastic shock-absorber element is articulated on a lug which is itself pivoted on the rigid frame and on which is pivoted one end of a rod of which the other end is pivoted on the lower connecting rod, the three axes of articulation on the lug being arranged so as to form a triangle in which the axis articulating the lower end of the shock-absorber to the lug is located between the two other axes and below these axes.